Weak at the Knees Part II

Chris McMaster takes us through why the ACL is injured, exploring the mechanism of the knee.

Brent Macaffer's shin can clearly be seen rotating inwards after a knock to the outside of the knee.

ACL injuries occur in a variety of different ways, but the principles remain the same. The ACL attaches to the inside of the tibia (shin), but to the outside of the femur (thigh).

This means that any force that moves the shin in and the thigh out will place the ACL under stress. Additionally, the ACL sits further forward on the shin and moves backwards up to the thigh. So any force that moves the shin forwards and the thigh backwards will place the ACL under stress.

If you add these scenarios together, you get the ACL’s “perfect storm”: the thigh rotating outwards and the shin rotating inwards. If this happens, the ACL is as good as gone.

ACLs often tear when a force is applied to the outside of a knee, with the foot planted on the ground. This force causes the rotation described above.

Additionally, this force places an enormous amount of stress on the medial collateral ligament (MCL), which runs along the inside of the knee.

On top of all this, the now loosened knee joint often squeezes the cartilage that sits inside the knee joint (meniscus), causing an additional meniscus tear.

When an ACL, MCL and meniscus are all teared in one single injury, this is called the unhappy triad (for obvious reasons). A recent example of this mechanism is the Brent Macaffer injury.

The basic mechanism of an ACL tear.

The ACL can also tear without any contact, when a player pivots and turns on a foot that is firmly planted on the ground. This motion can cause the same twisting as described above, without the need for a bump to the leg.

This mechanism is more likely to occur when the knee is bent, but even then it is fairly rare. A recent example is Mark Lecras’ injury at pre-season training.

After a player goes down with a suspected ACL tear, there are several ways that the club doctor or physiotherapist can tell whether the ACL is actually torn.

The most popular and easiest test to perform is called the anterior drawer test. If you remember from the last article, the ACL is not particularly important for stabilising the knee, however careful examination will show a small amount of instability after the ACL tears.

The ACL helps to stop the shin from moving forward relative to the thigh (remember, this role is vital in converting the pull of the hamstrings into rotation at the knee). If the ACL is torn, you should be able to move the shin forward relative to the thigh. This is a positive anterior drawer test.

Next up in the ACL series we’ll look into ACL surgery and recovery.

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